Air Compressor and Generator Buying Guide: Flow, Pressure, kVA and Energy Cost

In compressed air and power generation, the purchase price is typically only 15 to 20 percent of the ten-year total cost. Around 75-80 percent of a screw compressor cost is electricity, and for a generator it is fuel. The right choice is therefore made with kWh and litres, not with the sticker price.

The Numbers That Drive Compressor Selection

A compressor is specified by flow (m³/min or L/s), working pressure (bar) and duty profile. Selections made without measuring real air consumption are almost always oversized, and an oversized machine cycles between load and unload, where unloaded running is electricity you pay for and never use. The reliable method is a week-long air demand measurement showing peak, average and night-time consumption separately.

How Pressure Drives Energy Cost

The accepted rule is that every extra 1 bar of working pressure raises energy consumption by roughly 6-7 percent. A system needlessly producing 10 bar instead of 8 bar wastes a significant amount every year. Leaks are the other big loss: in a typical factory they account for 20-30 percent of total production, and a single 3 mm hole at 7 bar leaks about 0.4 m³/min.

Fixed Speed or Variable Speed

Variable speed drive (VSD) compressors save 20-35 percent compared with fixed-speed units in plants with fluctuating demand. Where demand is nearly constant, a fixed-speed machine is both cheaper and simpler. Atlas Copco, Kaeser and Ingersoll Rand all offer broad ranges in both categories.

Generators: Prime, Standby and Fuel Burn

Generators carry two ratings: standby (emergency use, limited hours per year) and prime (continuous duty). The same set may be rated 550 kVA standby and 500 kVA prime, so always confirm which figure the nameplate shows. As a rough guide, full-load fuel consumption is 0.25-0.28 litres per kWh, which means roughly 100-112 litres per hour at a 400 kW load. Sets built around Cummins, Perkins, Volvo Penta and Caterpillar engines have the widest service coverage.

Capacity Classes and Service Intervals

Equipment	Power range	Typical output	Service interval	Expected life
Screw compressor (fixed speed)	7.5-355 kW	1.1-60 m³/min @ 7.5 bar	Oil every 4,000 h	60,000-80,000 h
Screw compressor (VSD)	11-315 kW	20-100 percent modulation	Oil every 4,000 h	60,000-80,000 h
Piston compressor	2.2-30 kW	0.3-3 m³/min	Oil every 500-1,000 h	15,000-25,000 h
Refrigerant dryer	0.3-5 kW	+3 Celsius dew point	Annual cleaning	10-15 years
Diesel generator	10-2,500 kVA	0.25-0.28 L/kWh	Oil every 250-500 h	20,000-30,000 h

Used Compressor and Generator Checklist

1. Read total hours and loaded hours separately from the controller; a loaded ratio above 80 percent suggests a hard-worked machine.
2. Check the airend for oil leaks, high-temperature alarm history and bearing noise.
3. Ask for the last replacement dates of the oil separator, air filter and oil cooler.
4. Check for oil carryover in the delivered air with a simple paper test at the receiver outlet.
5. On generators inspect the radiator, batteries, jacket heater, alternator insulation resistance and fuel system for water and sediment.
6. Always load test a generator: run it at full load on a load bank for at least 30 minutes and watch temperature and voltage stability.
7. Confirm emission and noise compliance in advance, especially for urban installations.

How to Calculate Total Cost

A compressor ten-year cost is roughly: annual hours x installed kW x average load factor x electricity price x 10, plus maintenance and parts, plus purchase price. A 75 kW compressor running 6,000 hours a year at 70 percent average load consumes about 315,000 kWh annually. A 25 percent VSD saving on the same duty is about 78,000 kWh a year.

Heat recovery is the item most often ignored: 70-90 percent of the heat a screw compressor produces can be recovered as hot water or space heating. For plant distribution and switchgear, see electrical equipment.

Air Network and Pressure Drop

However efficient the compressor, savings disappear if the distribution network is badly designed. An undersized main line loses 0.3-0.5 bar every 100 metres, and that loss is compensated by running the compressor at higher pressure. A ring distribution layout instead of a dead-end line roughly halves pressure drop and limits the pressure dip when demand peaks.

Other measures worth taking: use wide-radius bends instead of sharp elbows, take branch connections from the top of the pipe so condensate does not reach the machine, fit a filter-regulator-lubricator set at each point of use, and run an annual leak survey. In a typical factory, one day of ultrasonic leak detection can recover 10-20 percent of annual compressed air energy.

New or Used?

For compressors and generators, annual running hours decide. For standby duty and low hours, used equipment makes a great deal of sense; for a main compressor feeding continuous production, a new high-efficiency machine repays itself on energy alone.

Criterion	New machine	Used machine
Initial investment	High	Substantially lower
Lead time	8-30 weeks after order	Usually 1-3 weeks
Warranty and service	Manufacturer warranty, commissioning included	Limited or dealer warranty
Technology and efficiency	VSD drive, heat recovery, current emission class engines	One or two generations behind
Spare parts	Secured for many years	Depends on model age
Depreciation	Steep loss in the first years	Much slower value loss
Finance and leasing	Suits leasing and incentives	Limited, needs appraisal

A new screw compressor delivers markedly more air per kilowatt-hour than an older generation thanks to variable speed drive and an efficient airend, and it arrives ready for a heat recovery connection. On the generator side, current emission class engines keep urban installations compliant and a remote monitoring module simplifies fuel tracking.

What to ask when buying new matters just as much as the used checklist:

• Lead time: the schedule from order confirmation to commissioning must be in writing.
• Warranty scope: duration, excluded parts and response time should be stated clearly.
• Service network: ask the distance to the nearest authorised service and its response time.
• Parts commitment: stock status and lead time for critical parts belong in the contract.
• Training and commissioning: confirm operator training and acceptance tests are included in the price.

Frequently Asked Questions

What compressor size do I need?

Add up the instantaneous air demand of every consumer, apply a diversity factor and add a 15-20 percent margin. As a rough conversion, 1 kW of compressor power delivers about 0.11-0.13 m³/min of air at 7.5 bar.

How large should the air receiver be?

A common rule for fixed-speed systems is 6 to 10 litres of receiver volume per L/s of compressor flow. A 50 L/s compressor therefore suits a 300-500 litre receiver. VSD compressors can work with smaller receivers.

How often should a standby generator be run?

Standby sets should be started at least monthly and ideally run 20-30 minutes under load. That exercise keeps the fuel system and battery healthy. Oil and filter changes should be done annually regardless of hours accumulated.

Is an air dryer really necessary?

Condensate in compressed air quickly damages pneumatic valves and paint booths. For general production a refrigerant dryer with a +3 Celsius dew point is enough. Food, pharma and outdoor lines need an adsorption dryer delivering -40 Celsius.

What is the riskiest part on a used generator?

The alternator winding and the control module. A set stored in a damp environment loses winding insulation, and repair is expensive. Discontinued control modules also mean long lead times when a fault occurs.

How fast does a new compressor pay back?

On a 75 kW class system running 6,000 hours a year, moving from an old fixed-speed machine to a current VSD unit saves around 25 percent, which usually covers the price difference in three to five years. At low running hours the payback stretches quickly.

To compare new and used screw compressors, dryers, air receivers and diesel generators, browse the power and compressed air systems category.